JP2010-249008A discloses an internal combustion engine in which a heat shield film is formed on a wall surface of a combustion chamber. This heat shield film has an anodized layer. The anodized layer has a porous structure. According to the porous structure, the anodized layer exhibits lower thermophysical properties in terms of thermal conductivity and thermal capacity per unit volume than any other component parts of the combustion chamber without the anodized layer.
According to such the properties, it is possible to make temperature of the surface on which the heat shield is formed follow that of working gas in the combustion chamber. Specifically, in an expansion stroke of the engine, it is possible to make the temperature of the surface follow rising temperature of the working gas. Also, in an intake stroke of the engine, it is possible to make the temperature of the surface follow that of intake air being relatively lower temperature. Hereinafter, such follow-up characteristics are also referred to as a “swing characteristic”. According to the heat shield film having the swing characteristic, it is possible to achieve both reduction in cooling loss and suppression of occurrence of knocking.
JP2003-307105A discloses an exhaust valve having a heat shield film formed on a surface of its parachute part. JPS61-3915U and JPS60-143127U disclose an internal combustion engine in which a heat shield film is formed in an inner wall of an exhaust manifold. These heat shield films are formed by a thermal spraying method. The heat shield films formed by the thermal spraying method have a porous structure like the anodized layer mentioned above. Therefore, the above-mentioned effect due to the swing characteristic are expected in these prior arts.
In general, an internal combustion engine mounted on a vehicle has a catalyst in an exhaust pipe of the engine. This catalyst has a function of purifying specific components contained in exhaust gas when the catalyst is in an activated state. Therefore, in order to use this function immediately after the start of the engine (especially immediately after cold start), it is important to warm up the catalyst rapidly by some means. Specifically, it is important to raise a bed temperature of the catalyst to an activation temperature range in a short time.
In this respect, if the heat shield films having swing characteristic are formed on each surfaces of component parts of an exhaust system including the wall surface of the combustion chamber, the surface of the parachute part and the inner wall of the exhaust manifold, it is possible to rapidly increase the temperature of the exhaust gas which flows into the catalyst. However, unlike the combustion chamber into which intake air flows in each cycle, the exhaust manifold or the exhaust pipe is able to exchange heat only with an outside. Therefore, if the heat shield film is formed on all of the inner walls of the exhaust manifold and exhaust pipe, it is expected that the temperature of the exhaust gas continues to be high even after the warming-up of the catalyst is completed. Therefore, there is a possibility that deterioration of the catalyst tends to progress easily.
The present disclosure addresses the above described problem, and an object of the present disclosure is, to provide an internal combustion engine which realizes early activation of a catalyst by using a heat shield film having the swing characteristic while suppressing deterioration of the catalyst.